Compared to Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) respectively, Single-Carrier Frequency Domain Equalization (SC-FDE) and Single-Carrier Frequency Division Multiple Access (SC-FDMA) show similar implementation complexity and robustness against highly dispersive channels, while generating a much lower peak-to-average power ratio (PAPR) waveform. SC-FDMA signals can be generated either in time domain or in frequency domain. However, in terms of implementation, the frequency-domain method is preferred, which utilizes the same system parameters and modules as those of the popular OFDMA to the maximum extent. As a multi-user extension version of SC-FDE and a variant of OFDMA, SC-FDMA has been adopted as the uplink communications scheme by the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE).
In practical implementation of SC-FDMA and SC-FDE systems, channel estimation is required for channel equalization and further detection. This is usually achieved by inserting separated RS-bearing SC-FDMA/SC-FDE symbols for each user and then interpolating/extrapolating the frequency-domain channel responses for data-bearing SC-FDMA/SC-FDE symbols.
A prior art apparatus 100 for generating a signal for transmission in a transmitter of an SC-FDMA system is illustrated in FIG. 1. As shown in FIG. 1, information bits to be communicated to the receiving side are first input to an encoder 101 and a modulator 102 to obtain a plurality of data symbol blocks. Each data symbol block, expressed as dn=[dn0, . . . , dnM−1]T, includes M data symbols dni, 0≦i≦M−1, where n represents the index of the symbol block in a sub-frame and T indicates transpose of a vector. Each data symbol block dn=[dn0, . . . , dnM−1]T is performed a serial-to-parallel (S/P) conversion in a S/P converter 103, and then is subject to an M-point discrete Fourier transformation (DFT) in a DFT unit 104, to produce a frequency-domain data sequence cn=[cn0, . . . , cnM−1]T. Then, in a reference signal (RS) inserter 105, a number of predefined frequency-domain RS sequences, each of which also includes M elements and is also represented by cn=[cn0, . . . , cnM−1]T, where n, however, indicates indices not used by data symbol blocks in a sub-frame, are multiplexed in time domain with the data sequences generated by the DFT unit 104. Then, elements of each cn (for all n) in a sub-frame are mapped to M sub-carriers scheduled by the base station in a sub-carrier mapping unit 106 to obtain a corresponding frequency-domain sequence, such as, for example, Cn=[0, . . . , 0, cn0, . . . , cnM−1, 0, . . . , 0]T. Subsequently, each Cn is performed N-point inverse Fast Fourier transformation (IFFT) in an IFFT unit 107 to produce a time-domain symbol block Dn including N elements, which is then performed a parallel-to-serial (P/S) conversion in a P/S converter 108. Finally, in a cyclic prefix (CP) inserter 109, a cyclic prefix is inserted before each of a plurality of time-domain symbol blocks outputted from the P/S converter 108. Thus, a digital discrete time signal for transmission is generated, which is then sent to a digital to analog (D/A) converter (not shown) for D/A conversion and a radio frequency (RF) section (not shown) of the transmitter to be subject to RF processing, and then is transmitted from an antenna (not shown) of the transmitter.
FIG. 2 is a schematic representation of the signal for transmission in a sub-frame generated by the apparatus 100 shown in FIG. 1. FIG. 3 is a more detailed representation of the generated signal for transmission, with each square representing a resource element (RE). Note that the cyclic prefixes are omitted from FIG. 3.
As shown in FIGS. 2 and 3, in two continuous time slots (one sub-frame), each user has two symbol blocks (e.g., n=3, 10 in the figures) that only bear RS, and 12 symbol blocks (e.g., n=0, 1, 2, 4, 5, 6, 7, 8, 9, 11, 12, 13 in the figures) that only bear data. Each RS-bearing symbol block occupies the entire transmission bandwidth assigned to each user (for example, all the 12 sub-carriers assigned to the user as shown in the figures) in order to maintain the SC property of an SC-FDMA waveform.
However, in a case of high Doppler spread (e.g., up to 350 km/h and 2 GHz carrier frequency), a system with such an RS arrangement shown in FIGS. 2 and 3 can not provide sufficient ability for tracking channel variation. The tracking ability may be improved by simply increasing the density of RS-bearing symbol blocks in time. However, this will result in an unbearable overhead.
Therefore, a need exits for an apparatus and method for generating a signal for transmission in a single-carrier communication system, which can provide sufficient ability for tracking channel variation with reasonably low overhead in a case of high Doppler spread.